feat: add Map type

[rhencke]

Summary

• Adds Map(value_type) type for map(string→T) Terraform attributes
• Wire format: ["map", <value_type>] — matches go-cty/OpenTofu encoding
• encode/decode delegate to value_type per value, matching List/Set pattern
• Handles None and Unknown sentinels consistently
• Full test coverage added to test_types.py

Motivation

Without Map, the only option for key→value attributes (e.g. environment variables, trigger maps) is NormalizedJson, which forces users to write jsonencode({...}) in HCL. With Map(String()) users can write natural HCL:

# Before
environment = jsonencode({ FOO = "bar" })

# After
environment = { FOO = "bar" }

Test plan

• test_map_encode — None/Unknown passthrough + encode/decode round-trip
• test_map_encode_delegates_to_value_type — value_type applied per value
• test_map_type_encoding — tf_type() for String/Number/Bool/nested types
• test_map_semantic_equality — dict equality, None/Unknown/empty cases

[hfern]

Thank you @rhencke for your contribution! 🎉 I've added some docs and a couple of e2e tests.

[rhencke]

Thank you! (tf is fantastic by the way)