Enhance UX for introducing new providers to configuration

[radeksimko]

Problem Statement

Users can today leverage completion capability for introducing a new provider block and new required_providers entry to the configuration, but the experience as whole isn't as seamless as it could be.

Users generally have to follow a particular sequence of steps:

1. Find terraform block
2. Find required_providers block within
3. Add new entry to that block
4. Find correct provider address to use, such as hashicorp/aws
5. Find suitable version constraint to use, such as ~>3.53
6. Add provider block to configure the provider

Ideally users shouldn't need to follow any particular sequence in most cases and all users should need to know is the name of the provider - such as aws to get a working configuration as if all of the above steps were followed.

Expected User Experience

TODO

Proposal

• Search registry providers via completion terraform-ls#194
• New Code Action: Generate missing required_providers entry terraform-ls#480
• Add links to provider sources in required_providers block terraform-ls#440
• Install/upgrade provider via code lens #679 / Install/upgrade provider via required_providers code lens terraform-ls#299
• Add required_providers entry on provider completion terraform-ls#370
• Provide completion for required_providers terraform-ls#371
• Add links to provider sources in required_providers block terraform-ls#440
• Convert legacy provider addresses via Registry API terraform-ls#458
• Completion for required_providers source & version #676