Use SHA2 and PSS for client assertion

[bgavrilMS]

Fixes #4428

Changes proposed in this request

• use SHA2 and PSS for all authority types except Generic, dSTS and ADFS.
• refactor to use string operations instead of JSON serialization for creating assertions

Testing

Performance impact

Using strings operations instead of JSON serialization leads to a small ~1KB memory improvement while the performance is marginally better, ~10% gain.

OLD

Method	UseX5C	UseExtraClaims	Mean	Gen0	Allocated
SimpleAssertion	False	False	690.4 us	0.9766	8 KB
SimpleAssertion	False	True	683.2 us	0.9766	10.35 KB
SimpleAssertion	True	False	680.3 us	3.9063	25.43 KB
SimpleAssertion	True	True	705.1 us	3.9063	27.27 KB

NEW

Rows 5-8 use SHA2

Method	UseSha2	UseX5C	UseExtraClaims	Mean	Gen0	Allocated
SimpleAssertion	False	False	False	687.2 us	-	7.06 KB
SimpleAssertion	False	False	True	658.0 us	0.9766	9.08 KB
SimpleAssertion	False	True	False	640.8 us	2.9297	23.56 KB
SimpleAssertion	False	True	True	657.4 us	3.9063	25.59 KB
SimpleAssertion	True	False	False	662.6 us	0.9766	8.27 KB
SimpleAssertion	True	False	True	657.8 us	0.9766	10.31 KB
SimpleAssertion	True	True	False	670.3 us	3.9063	24.78 KB
SimpleAssertion	True	True	True	678.4 us	3.9063	26.8 KB

Documentation

• All relevant documentation is updated.