llms: add llama3 models to bedrock

AWS announced that Meta’s Llama 3 models are now available in Amazon Bedrock.

[X] Read the Contributing documentation.
[X] Read the Code of conduct documentation.
[X] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
[X] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
[X] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
[X] Describes the source of new concepts.
[X] References existing implementations as appropriate.
[X] Contains test coverage for new functions.
[X] Passes all golangci-lint checks.

tmc / langchaingo