Return spec in backstage_entities data source

[tdabasinskas]

The PR adds support for spec attribute for entities returned by backstage_entities data source and fixes #101.

It might not be ideal as it is returned as JSON string instead of a proper object. I wasn't able to achieve this with dynamic types so that's the reason I chose JSON. jsondecode function can be used to decode the object and get the specific values.

Usage example:

terraform {
  required_providers {
    backstage = {
      source = "tdabasinskas/backstage"
      version = "2.4.0-rc1"
    }
  }
}

provider "backstage" {
  base_url = "https://demo.backstage.io"
}

data "backstage_entities" "example" {
  filters = []
}

output "example" {
  value = jsondecode(data.backstage_entities.example.entities[0].spec)["profile"]["email"]
}

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 76.55%. Comparing base (64f54d2) to head (aa4803c).

Files	Patch %	Lines
backstage/data_source_entities.go	33.33%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   76.83%   76.55%   -0.28%     
==========================================
  Files          11       11              
  Lines        1390     1399       +9     
==========================================
+ Hits         1068     1071       +3     
- Misses        237      242       +5     
- Partials       85       86       +1     

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