Make max number of subnet mechanisms configurable

[gztensor]

Description

• Add admin extrinsic to set max number of subnet mechanisms.
• Ensure max allowed uids * current mechanism count never exceed 256 on a subnet.
• Set the SubtensorInitialMaxAllowedUids from 4096 to 256.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[l0r1s]

Could be nice to have a few unit tests, wdyt?

[l0r1s]

It needs to be reset on dereg?

[sam0x17]

can we clean this one up too @l0r1s ?