SimpleDN: adopt broader RFC4514-compatible parsing

[keaz]

/Users/kasunranasinghe/Projects/Rust/simple-ldap/src/simple_dn.rs:128-246 supports a constrained DN grammar.

Problem

Current parser rejects many valid LDAP DNs (multivalued RDNs, broader RDN key/value char sets, and some escaping scenarios), limiting use for real-world directories.

Proposal

Adopt broader RFC4514-compatible parsing behavior (or explicitly document strict-mode behavior).

Acceptance criteria

• Support common escaped forms and safer parsing of values.
• Ensure display/parsing round-trip for typical RFC-compatible DNs.
• Update SimpleDN docs/comments to clearly state parser strictness/compatibility mode.
• Add tests for previously rejected-but-common valid DN forms.

[Iizuki]

No one has requested anything of this sort. Can we close this?