Support for overriding instance metadata endpoint

[wjones127]

Which issue does this PR close?

Closes #2802.

Rationale for this change

Support getting credentials in AWS ECS.

What changes are included in this PR?

As suggested, allows user to set instance metadata endpoint. Extract from the documented environment variable.

Are there any user-facing changes?

Yes, new methods on the AWS builder. I have added docs here and verified the rendered output looks good.

[wjones127]

(whoops ran cargo test and not cargo test --features aws 🤦 )

[tustvold]

Just some minor nits

[ursabot]

Benchmark runs are scheduled for baseline = 6477db1 and contender = 3999c77. 3999c77 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java