Add support for platform security TLS actions

@marclop commented on [Fri Mar 23 2018](https://github.com/elastic/cloud-cli/issues/265)

## Current Behavior
There's no support to obtain / Upload TLS certificates.

## Possible Solution

### Show certificates for a service
`ecl platform certificate show `

### Upload certificates for a service
`ecl platform certificate upload --chain chain.pem --service `

## Context

we should support all of the actions the API spec supports.

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@ppapapetrou76 commented on [Tue Feb 23 2021](https://github.com/elastic/cloud-cli/issues/265#issuecomment-784157111)

@marclop I believe this one should be implemented on `ecctl` because the supporting API is public, no?
I'm happy to contribute in that case. WDYT?

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@ppapapetrou76 commented on [Tue Feb 23 2021](https://github.com/elastic/cloud-cli/issues/265#issuecomment-784158424)

And I think it's this one https://www.elastic.co/guide/en/cloud-enterprise/current/set-tls-certificate.html right?

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@karencfv commented on [Wed Feb 24 2021](https://github.com/elastic/cloud-cli/issues/265#issuecomment-784705257)

Heya @ppapapetrou76, yes, looks like that command is meant for ecctl. Looking at the API endpoint, and to keep API parity with the CLI I would like to change the format of the command as described on the issue here to:
```go
ecctl platform configuration tls --service
```

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@cjwilburn commented on [Fri Mar 26 2021](https://github.com/elastic/cloud-cli/issues/265#issuecomment-808437194)

@ppapapetrou76 @karencfv is there a need to keep this open if the changes need to happen elsewhere?