update path validator to enforce no beginning or ending of the separator

[colin-axner]

Description

Updates path validator to enforce that no path can begin or end with the separator. Previously both were allowed, but I believe this goes against the ICS spec which states:

The separator "/" is used to separate and concatenate two identifiers or an identifier and a constant bytestring. Identifiers MUST NOT contain the "/" character, which prevents ambiguity.

IMO beginning or ending with the separator is ambiguous to whether the separator is apart of the identifier name (which isn't allowed) or if the second identifier is an empty string (again not allowed). It is also more logical that a separator is always used to separate either two identifiers or an identifier and a constant byte string.

continuation of #6227

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[alexanderbez]

utACK 🎉

[codecov]

Codecov Report

Merging #6233 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6233      +/-   ##
==========================================
- Coverage   54.76%   54.75%   -0.02%     
==========================================
  Files         447      447              
  Lines       26904    26896       -8     
==========================================
- Hits        14734    14726       -8     
  Misses      11126    11126              
  Partials     1044     1044              

[fedekunze]

ACK 👍 good catch