HttpSemanticConventions - update helper class

[TimothyMothra]

Towards #4484

When I started on this, I was working off the older version of the spec in the old repo.
The current version of the spec in the new repo changed the environment variable to a comma-separated list.

SHOULD introduce an environment variable OTEL_SEMCONV_STABILITY_OPT_IN in the existing major version which is a comma-separated list of values.
https://github.com/open-telemetry/semantic-conventions/tree/v1.21.0/docs/http

Changes

• refactor HttpSemanticConventionsHelper
  • rewrite to parse a CSV
• updated several projects to include Shared\Shims\NullableAttributes
• refactor unit tests.
  • replaced InlineData with MemberData so each test can use the same collection of test cases.
  • added additional csv test cases

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

Note about duplicate values in the Environment Variable.

Edit: 8/14: The spec has been updated to specify if both http and http/dup are specified, http/dup will take precedence.
open-telemetry/semantic-conventions#249

As of today, the only valid values are http (emit new attributes) and http/dup (emit both new and old attributes).
The spec doesn't define how to handle a scenario where a user provides both values in the CSV.
I opened an issue to get clarity: open-telemetry/semantic-conventions#240

In this scenario, Java takes a user-friendly approach of the approach of enabling both new and old attributes. I think we should do the same here.
https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/32c5d4c26708ccaa20221bb6d687f56c9efde979/instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/internal/SemconvStability.java#L26-L39

[codecov]

Codecov Report

Merging #4765 (155e9c1) into main (5784b45) will increase coverage by 0.02%.
The diff coverage is 86.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4765      +/-   ##
==========================================
+ Coverage   82.27%   82.29%   +0.02%     
==========================================
  Files         314      314              
  Lines       12923    12930       +7     
==========================================
+ Hits        10632    10641       +9     
+ Misses       2291     2289       -2     

Files Changed	Coverage Δ
src/Shared/HttpSemanticConventionHelper.cs	82.35% <86.66%> (+2.35%)	⬆️

... and 5 files with indirect coverage changes