spiffe: add support for spiffe bundle format (#36190)

Commit Message: Adds alternative to "trust_domains" config for the
spiffe validator—"trust_bundle_map".

Additional Description:

[#35567](https://github.com/envoyproxy/envoy/issues/35567)
trust_bundle_map points to a local file containing a [SPIFFE bundle
map](https://docs.google.com/document/d/13KHycYIfRC-g42aEfo4_4inF_WauCXOBgZAcUGuxdgs/edit#heading=h.o2sg9lu1e74v).
A file watcher is set up to trigger refreshes to the SPIFFE data when
this file is modified. SPIFFE refresh hint and sequence number are
currently ignored.

Risk Level: medium
Testing: WIP
Docs Changes: TBD
Release Notes: TBD

---------

Signed-off-by: Brian Sonnenberg <bsonnenberg@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ c60d428b3d0ed568a96d30f4c91f77843a308c19

envoy/extensions/transport_sockets/tls/v3/tls_spiffe_validator_config.proto

@@ -57,4 +57,11 @@ message SPIFFECertValidatorConfig {
 
   // This field specifies trust domains used for validating incoming X.509-SVID(s).
   repeated TrustDomain trust_domains = 1 [(validate.rules).repeated = {min_items: 1}];
+
+  // This field specifies all trust bundles as a single DataSource. If both
+  // trust_bundles and trust_domains are specified, trust_bundles will
+  // take precedence. Currently assumes file will be a SPIFFE Trust Bundle Map.
+  // If DataSource is a file, dynamic file watching will be enabled,
+  // and updates to the specified file will trigger a refresh of the trust_bundles.
+  config.core.v3.DataSource trust_bundles = 2;
 }