Identifier validation inconsistent with ICS024

[adizere]

Summary of Bug

ICS 024 defines several criteria for an identifier to be valid:
https://github.com/cosmos/ics/tree/master/spec/ics-024-host-requirements#paths-identifiers-separators

Identifiers MUST be non-empty (of positive integer length).
Identifiers MUST consist of characters in one of the following categories only:

• Alphanumeric
• ., _, +, -, #
• [, ], <, >

Our validation function implementation is more strict than the above requirements (and possibly buggy).

There are a couple of issues:

1. we prevent characters such as ., _, +, -, #
2. we prevent numbers (each character has to be both alphanumeric and lowercase, and numbers are not lowercase)

https://github.com/informalsystems/ibc-rs/blob/96ee9735e050a87b09b3ba7544081537953d64ba/modules/src/ics24_host/validate.rs#L41-L43

Can we align validation with ICS024? Or is there a reason why we have a more strict validation on identifiers?

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[adizere]

After a closer look:

It seems that the identifier validation requirements changed more recently (April 27) than our code (originally ~March):

cosmos/ibc@dc4ecb8