Support trimming trailing slashes via a mount tuneable to support CMPv2

[sgmiller]

Description

CMPv2 specifies that the cmp path may or may not end in a trailing slash.
Vault's request processing has a requirement that they do not for POST. To
make this work, add a tuneable to the mount that will trim slashes if
enabled. All existing mounts will default to off, preserving the existing
behavior. CMPv2 capable PKI mounts will enable this.

TODO only if you're a HashiCorp employee

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  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
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  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
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