Use SHA256 based on new env variable CONNECT_VBMS_SHA256

[lisac-usds]

Introduces support for using SHA256 for the signature and digest algorithms.

Selects SHA256 if env variable CONNECT_VBMS_SHA256 == "True"; else uses SHA1 (previous behavior).

The env variable CONNECT_VBMS_SHA256 is introduced in the deployment scripts in appeals-deployment#1424.

This is a variation of #177.

[lisac-usds]

TODO:

• add unit test that checks the SHA256 flow
• simplify the equality check on the new env variable

(7 May update: simplified the equality check and added issue #182 for the unit test)