Update resource grafana_data_source

[mlclmj]

Summary

Makes long-overdue updates to the data source resource to introduce additional datasource types that can be fully configured with this provider including:

• Elasticsearch
• Graphite
• OpenTSDB
• Prometheus
• MySQL
• MSSQL
• Postgres

This change also adds better testing for the types of datasources that can be added.

Details

• Updated the upstream library to v0.3.1
• Reordered schema fields into alphabetical order for readability/maintainability
• Added table tests for the various data source types supported. Note that the Grafana API doesn't actually validate the presence of any of these fields or their values, however this is valuable in making sure that the upstream library is parsing and submitting values to Grafana as expected.
• As part of this work, the access_key and secret_key fields of the secure_json_data block are now marked as optional rather than required. Previously this was in place as the only datasource type using the secret_json_data block was Cloudwatch, though this is no longer the case. This shouldn't result in a breaking change for anyone configuring Cloudwatch datasources through this provider.
• Vendor folder was updated using go mod vendor

Resolves #22
Resolves #42

[mlclmj]

Resolves #54

[mlclmj]

Resolves #68

[tonglil]

Nice set of changes, LGTM

[moseskt]

@mlclmj any eta for full support for configuring Azure Monitor via terraform?