Add support for multiple bearer tokens

[bchen528]

Description

This PR introduces support for multiple bearer tokens in the bearer token auth extension.

Testing

• Unit tests
• Manually tested within custom otel collector to verify that multiple bearer tokens work

Documentation

• Added changelog entry
• Added configuration documentation for bearer token authentication extension

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: bchen528 / name: Becky Chen (d470456, 5c42e01, 73ae81a, 354ad9b, 8bcaf19, 66d31c4, ef127d2, b9a0382, 2e57066, 12264df, 6707cc2, 1da31b4, 7d79a9b, d722413, e7e11c7, 1b6af86)

[jpkrohling]

The validate function should be changed to prevent users from setting both "token" and "tokens".

[jpkrohling]

This can be merged when CI is green. Thanks!

[jpkrohling]

This PR has been failing at the linting stage:

/home/runner/work/opentelemetry-collector-contrib/opentelemetry-collector-contrib/.tools/golangci-lint run --allow-parallel-runners --build-tags integration --timeout=30m --path-prefix bearertokenauthextension
Error: bearertokenauthextension/bearertokenauth_test.go:339:9: G306: Expect WriteFile permissions to be 0600 or less (gosec)
	err := os.WriteFile(filename, []byte(fileContent), 0644)
	       ^

Please, run the linter locally and fix the issues it reports.

[bchen528]

Thanks @jpkrohling! Done 👍

$ make lint
running /vagrant/opentelemetry-collector-contrib-becky/.tools/misspell -error
/vagrant/opentelemetry-collector-contrib-becky/.tools/golangci-lint run --allow-parallel-runners --build-tags integration --timeout=30m --path-prefix bearertokenauthextension

$ make test
cd /vagrant/opentelemetry-collector-contrib-becky/internal/tools && GOOS="" GOARCH="" go build -o /vagrant/opentelemetry-collector-contrib-becky/.tools/gotestsum -trimpath gotest.tools/gotestsum
go: downloading gotest.tools/gotestsum v1.12.0
go: downloading github.com/dnephin/pflag v1.0.7
go: downloading github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510
go: downloading github.com/bitfield/gotestdox v0.2.2
/vagrant/opentelemetry-collector-contrib-becky/.tools/gotestsum --rerun-fails=1 --packages="./..." -- -race -timeout 600s -parallel 4 --tags=""
∅  internal/metadata
✓  . (26.095s)

DONE 26 tests in 73.562s