Update to latest AWS SDK

Also:

  - Simplify some CloudWatch code, allowing the removal of `chrono` and
    a related AWS type conversion crate
  - Bump MSRV to 1.70.0
  - Switch to using short struct initialization where possible
  - Move the API Usage section out of the `README.md` and into its own
    file, `API_USAGE.md`

API_USAGE.md

@@ -0,0 +1,95 @@
+# API Usage
+
+Each of the methods for obtaining bucket sizes have different costs within AWS
+as they will make differing numbers of API calls, and each API has its own
+pricing model which may also vary by region.
+
+The following is a rough guide to what `s3du` is doing so you can gauge what
+the API usage may be.
+
+### CloudWatch
+
+AWS CloudWatch is the cheapest method of running `s3du`, at the cost of some
+accuracy.
+
+The CloudWatch mode of `s3du` will use at least 1 API call to perform the
+`ListMetrics` call and at least 1 API call per S3 bucket for the
+`GetMetricStatistics` call.
+
+The reason these are listed as "at least 1" is because the API call results
+could be paginated if the results lists are sufficiently long. `ListMetrics`
+will paginate after 500 results while `GetMetricStatistics` will paginate after
+1,440 statistics.
+
+As a basic example, getting bucket sizes for an AWS account with 4 S3 buckets
+in it should use 5 API calls total. 1 `ListMetrics` call to discover the
+buckets and 4 `GetMetricStatistics` calls (one for each bucket).
+
+### S3
+
+AWS S3 is a more expensive, but more accurate, method of listing bucket sizes.
+
+The S3 mode of `s3du` will use 1 API call to perform the `ListBuckets` API
+call, 1 API call per listed bucket to `GetBucketLocation` to discover its
+region, 1 API call per listed bucket to `HeadBucket` to make sure we have
+access to list the objects, and:
+
+  - at least 1 call to `ListMultipartUploads`, at least 1 call to
+    `ListObjectVersions`, and at least 1 call to `ListParts` if in-progress
+    multipart uploads are found in the `All` object mode
+  - at least 1 call to `ListObjectsV2` per-bucket in the `Current` object
+    (default) mode
+  - at least 1 call to `ListObjectVersions` per bucket in the `NonCurrent`
+    object mode
+  - at least 1 call to `ListMultipartUploads` per-bucket in the `Multipart`
+    mode with at least 1 call to `ListParts` if any in-progress multipart
+    uploads are found
+
+Each of the API calls listed above will return 1,000 objects maximum, if your
+bucket has more objects than this, pagination will be required.
+
+For example, let's say we're running in S3 mode getting the sizes of `current`
+object versions and our AWS account has 2 buckets.
+`bucket-a` (no versioning enabled) has 10,000 objects and `bucket-b`
+(versioning enabled) has 32,768 object versions of which 13,720 are current
+versions and 19,048 are non-current versions. There is also an in-progress
+multipart upload with 2 parts uploaded in `bucket-a`. This would mean:
+
+  - 1 API call to `ListBuckets` for bucket discovery
+  - 2 API calls to `GetBucketLocation` for region discovery, 1 for each bucket
+  - 2 API calls to `HeadBucket` to check we have access, 1 for each bucket
+  - 10 API calls to `ListObjectsV2` for `bucket-a`
+  - 14 API calls to `ListObjectsV2` for `bucket-b`
+
+for a total of 29 API calls.
+
+If we were to run `s3du` against the same account a second time, but ask for
+the sum of `all` object versions, we'd get the following:
+
+  - 1 API call to `ListBuckets` for bucket discovery
+  - 2 API calls to `GetBucketLocation` for region discovery, 1 for each bucket
+  - 2 API calls to `HeadBucket` to check we have access, 1 for each bucket
+  - 10 API calls to `ListObjectVersions` for `bucket-a`
+  - 33 API calls to `ListObjectVersions` for `bucket-b`
+  - 1 API call to `ListMultipartUploads` for `bucket-a`
+  - 1 API call to `ListMultipartUploads` for `bucket-b`
+  - 1 API call to `ListParts` for `bucket-a`
+
+for a total of 51 API calls.
+
+A third run of `s3du` against the same account but asking for the sum of
+`non-current` object versions would result in the following:
+
+  - 1 API call to `ListBuckets` for bucket discovery
+  - 2 API calls to `GetBucketLocation` for region discovery, 1 for each bucket
+  - 2 API calls to `HeadBucket` to check we have access, 1 for each bucket
+  - 1 API calls to `ListObjectVersions` for `bucket-a`
+  - 33 API calls to `ListObjectVersions` for `bucket-b`
+
+for a total of 39 API calls.
+
+You will notice that the number of API calls to `ListObjectVersions` for
+`bucket-b` are the same across both the `all` and `non-current` object versions
+requests, this is because any filtering for current vs. non-current objects in
+these scenarios must be done by `s3du`. The `ListObjectVersions` API does not
+let us specify which object versions we'd like to retrieve.

CHANGELOG.md

@@ -4,8 +4,9 @@
 
   - Switch from [Rusoto] to the official [AWS SDK Rust]
   - Update [clap] to 4.x
-  - Bump MSRV to 1.65.0
+  - Bump MSRV to 1.70.0
   - Switch from [lazy_static] to [once_cell]
+  - Simplify code in CloudWatch mode removing the need for [chrono]
 
 ## v1.1.0