Ability to select a SVID when multiple are available

[strideynet]

Today, if multiple SVIDs are received by the helper, it will select the first one. This is as per https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE_Workload_API.md#53-default-identity which states that:

The default identity is the first in the svids list returned in the X509SVIDResponse message.

We should add CLI flags to allow this behaviour to be modified, and to select a SVID based on it's "hint" or based on the SPIFFE ID contained within.