Add WIT-SVID TTL configuration and entry field

[sorindumitru]

Similarly to X509 and JWT SVIDs we need to have the WIT-SVID TTL specifiable both as a default value and override-able at the registration entry level.

[angabini]

I’m interested in picking this up.
I see the related SDK changes happening in spiffe/spire-api-sdk#90.
Is this ready for a PR, or are we waiting for the SDK PR to land first? May I be assigned?