[Integrations/ita] Add RA-TLS/SecretProv libs for Intel Trust Authority

[AI-Memory]

This commit adds the sources, documentation and tests for libra_tls_verify_ita.so and libsecret_prov_verify_ita.so libraries, used for integration of Gramine with ITA.

This PR is largely based on the [Integration/azure] contributed by Dmitrii and a patch from Scott.

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This change is 

[dimakuv]

This PR is a bit hacky; many things are missing in this PR (like verifying the signature of the JWT token), many things are redundant (like parsing the public key of the cert, since it's not used in the PR).

The general idea of the PR is good. I will take over this work (ITA + Gramine integration), and will submit a PR similar to this one, but with all details flashed out. I will add @bigdata-memory as a co-author in the new PR. I hope it's fine with @bigdata-memory.

My analysis can be found in #73.

[dimakuv]

A quick note.

The statement in this PR's README:

Note that the library does not use the following SGX-enclave-status environment variables:
RA_TLS_ALLOW_OUTDATED_TCB_INSECURE, RA_TLS_ALLOW_HW_CONFIG_NEEDED and RA_TLS_ALLOW_SW_HARDENING_NEEDED. This is because ITA will only generate a JWT for the SGX enclave if the enclave's TCB level matches the "TCB baseline" specified in the used ITA policy. In other words, ITA takes the responsibility away from the Gramine user and decides about the allowed security status of the SGX enclave based on its policy and not based on the aforementioned RA-TLS environment variables.

This statement is wrong. It was copied from MAA, where the statement was true. However, the ITA does not take the responsibility away from the user but actually reports the TCB level/status in the JWT field attester_tcb_status. In fact, there is a bunch of JWT fields, under the class "Attester claims", that must be verified by the end user, see here.