SPIFFE: support JWK set bundle format

[kyessenov]

Implement support for SPIFFE bundle format in the SPIFFE validator: https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE_Trust_Domain_and_Bundle.md#4-spiffe-bundle-format

This allows de-coupling of the identity system from the xDS control plane, and the trust bundles can be updated independently.

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This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted" or "no stalebot". Thank you for your contributions.

[briansonnenberg]

WIP PR

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted" or "no stalebot". Thank you for your contributions.