Introduce a TRUST_TOKEN_METHOD hook to select TRUST_TOKEN variations.

For now, it does nothing. This will make it easier to transition between
versions of the experiment while the protocol evolves.

Update-Note: Pass TRUST_TOKEN_experiment_v0() into any existing code
that now needs a TRUST_TOKEN_METHOD.

Change-Id: I434e18c794ab30545e367eb902e434e6311b7497
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/41066
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
diff --git a/include/openssl/base.h b/include/openssl/base.h
index 7218967..7f6acce 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -425,6 +425,7 @@
 typedef struct trust_token_st TRUST_TOKEN;
 typedef struct trust_token_client_st TRUST_TOKEN_CLIENT;
 typedef struct trust_token_issuer_st TRUST_TOKEN_ISSUER;
+typedef struct trust_token_method_st TRUST_TOKEN_METHOD;
 typedef struct v3_ext_ctx X509V3_CTX;
 typedef struct x509_attributes_st X509_ATTRIBUTE;
 typedef struct x509_cert_aux_st X509_CERT_AUX;