Add native Go fuzz drivers to enable OSS-Fuzz integration

[xingyaner]

Describe the bug
The go-ldap project is integrated with OSS-Fuzz, but the repository currently ships no fuzzer entry points. OSS-Fuzz's compile_native_go_fuzzer requires Fuzz* functions with the func FuzzXxx(f *testing.F) signature to exist inside the target package; without them, the OSS-Fuzz build cannot produce any fuzzer binaries for this project.

To Reproduce
Build the project via OSS-Fuzz :
python3 infra/helper.py build_fuzzers --sanitizer address --engine libfuzzer --architecture x86_64 go-ldap
The build fails because the functions do not exist in the package:
Could not find the function: func FuzzParseDN(f *testing.F)

Expected behavior
The build succeeds and produces the fuzzer binaries fuzz_parse_dn, fuzz_decode_escaped_symbols and fuzz_escape_dn. Additionally, the same Fuzz* functions can be run locally with Go's native fuzzing: go test -fuzz=FuzzParseDN ./v3.

Code snippets
If applicable, add code snippets to help explain your problem.

Additional context

• The three functions cover the DN parsing and escaping logic in v3/dn.go (ParseDN, decodeString, encodeString).
• This change goes together with a change to the OSS-Fuzz project config (projects/go-ldap/build.sh), which will be updated in the oss-fuzz repository to run the build inside the v3 module (cd v3 && go get ...) and point the fuzzer targets at the /v3 package path. Both sides are required for the OSS-Fuzz build of go-ldap to succeed.
• check_build already passes with these drivers.

[xingyaner]

This is the PR. #623