Add container runtime to container conventions

[rakyll]

This allows users to identify the engine and possible issues with the runtime when debugging.

[rakyll]

Addressed the feedback, please take a look.

[arminru]

Actually it would probably be better to use an enum like we did elsewhere, for example here:

opentelemetry-specification/semantic_conventions/resource/cloud.yaml

Lines 7 to 22 in 7033091

	- id: provider
	type:
	allow_custom_values: true
	members:
	- id: AWS
	value: 'aws'
	brief: 'Amazon Web Services'
	- id: Azure
	value: 'azure'
	brief: 'Microsoft Azure'
	- id: GCP
	value: 'gcp'
	brief: 'Google Cloud Platform'
	brief: >
	Name of the cloud provider.
	examples: 'gcp'

opentelemetry-specification/specification/resource/semantic_conventions/cloud.md

Lines 22 to 28 in 7033091

	`cloud.provider` MUST be one of the following or, if none of the listed values apply, a custom value:
	| Value | Description |
	|---|---|
	| `aws` | Amazon Web Services |
	| `azure` | Microsoft Azure |
	| `gcp` | Google Cloud Platform |

This will allow constant classes or helpers generated from the YAML definitions to provide constants for these well-known engines in order to ensure we always get the same values. If users have to provide the string values themselves, we will probably get crio in one case and CRI-O in another.
@rakyll Do you think this would make sense?