Add make command to upgrade DCL, use it to see if it works.

[nat-henderson]

Minor tpgtools usability enhancement to make it easier to upgrade to the newest version of the DCL. Inspired by kcctools' similar version of the same thing.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[nat-henderson]

Might crash in generation - not sure how prickly go is about tabs and spaces and formatting in there. Can't seem to get a tab in sed in the makefile, not sure why, think it's using a different sed than my system sed.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 3 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 3 files changed, 4 insertions(+), 2 deletions(-))

[nat-henderson]

Works On My Machine ™️ :)

[rileykarson]

What does this do vs running go get github.com/GoogleCloudPlatform/declarative-resource-client-library@latest? Will it just avoid getting the new version on the user's machine?

[nat-henderson]

Yep, that's all!

[nat-henderson]

Oh, also it doesn't depend on what version of go you're running or your setting of GO111MODULES (or whatever that env var is called)

[rileykarson]

Echoing to go.sum is allowed because you can have multiple versions of a library in there, right? So running go mod tidy on a downstream and copypasting up will clean those up when there's a lot of them.

[nat-henderson]

Yes!

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeForwardingRule_update You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=205772

[nat-henderson]

Great, this works!