[Feature]: Client should provide environment (instead of platform) setting

[gcatanese]

Feature summary

Add environment (test or live) attribute to the Client object

Problem statement

The Client platform attribute defines the Environment (test or live) to be used when making the API requests.
All other Adyen libraries instead use the environment attribute.

Proposed solution

Consider the following:

• deprecate Client platform attribute (mark to be removed in the future)
• add a new Client environment attribute
• update unit tests and README accordingly

Alternatives considered

No response

Additional context

No response