feat: add sort_by=name option to amazon-ami datasource

[juan-rodriguez-gracia]

Summary

Closes #650

Adds a sort_by = "name" option to the amazon-ami data source that selects the AMI with the lexicographically highest name, instead of relying solely on creation date (most_recent = true).

This solves a real-world problem where vendors publish patches for multiple AMI families in parallel (e.g. SUSE SLES SP6 and SP7), causing most_recent = true to select the wrong version — a newer patch for an older series can have a later creation date than the target version.

Changes

• common/step_source_ami_info.go — Add LatestByNameAmi helper that sorts a slice of images by name and returns the lexicographically highest
• common/ami_filter.go — Add GetFilteredImages method (alongside existing GetFilteredImage) that returns the full image slice without selecting one; callers handle selection
• common/test_helper_funcs.go — Add FakeAccessConfigWithEC2Client test helper for injecting mock clients in datasource tests
• datasource/ami/data.go — Add sort_by field to Config, validation in Configure(), and branching logic in Execute()
• datasource/ami/data.hcl2spec.go — Add sort_by attribute to the generated HCL2 spec

Behaviour

• sort_by = "name" selects the AMI with the lexicographically highest name
• Only "name" is a valid value; any other value produces a config error
• If both sort_by = "name" and most_recent = true are set, sort_by takes precedence silently
• AmiFilterOptions and all builder source_ami_filter blocks are unchanged

Example

data "amazon-ami" "source_os" {
  filters = {
    name                = "suse-sles-15-sp*-v202*-hvm-ssd-x86_64"
    root-device-type    = "ebs"
    virtualization-type = "hvm"
  }
  most_recent = true
  sort_by     = "name"   # selects sp7 over sp6 regardless of creation date
  owners      = ["013907871322"]
  region      = var.aws_region
}

Test plan

• TestLatestByNameAmi_ReturnsLexicographicallyLast — 3 AMIs with different SP versions → returns SP7
• TestLatestByNameAmi_SingleImage — single image → returns it unchanged
• TestLatestByNameAmi_IdenticalNames — identical names → no panic
• TestGetFilteredImages_ReturnsAllImages — mock returns 2 images → full slice returned
• TestGetFilteredImages_EmptyResultReturnsError — empty result → error
• TestGetFilteredImages_APIErrorPropagated — API error → error propagated
• TestDatasourceConfigure_SortByName_Valid — sort_by = "name" → no error
• TestDatasourceConfigure_SortByInvalid_Error — sort_by = "creation_date" → config error
• TestDatasourceConfigure_SortByAndMostRecent_NoError — both set → no error
• TestDatasourceExecute_SortByName_SelectsLexicographicallyLast — SP6 (newer date) vs SP7 (older date) → SP7 selected

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