Add nightly dependency bump workflow

[ezhang6811]

Issue #, if available:

Description of changes:
Add a workflow nightly-build.yml with following actions:

• creates or checks out existing nightly-dependency-updates branch
• checks for newest versions of dependencies in opentelemetry-python and opentelemetry-python-contrib, bumps versions if applicable, and creates/updates PR
• search for releases with breaking changes in release notes, and links in the PR
• runs main build checks on the branch, publishes a failure metric based on the outcome.

Example run: https://github.com/aws-observability/aws-otel-python-instrumentation/actions/runs/19088083503
PR created: #533

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[ezhang6811]

I just realized: I updated main build so that it also triggers when pushes are made to the nightly-dependency-updates branch so that updates to the nightly build PR can trigger main build again. If this is the case, do we even need to run main build in the nightly build workflow itself, or is that redundant?

[thpierce]

Not a complete review, just some initial thoughts.