Align the input handling with the CLI

[Pijukatel]

1. Storage client raises an error if there are multiple competing files for the same key.

For example, if there is INPUT and INPUT.json, then it should not try to guess which one should be used, but raise an error.

2. Actor takes the input key name from the env var

Ordered from highest priority to lowest:
ACTOR_INPUT_KEY
APIFY_INPUT_KEY
CRAWLEE_INPUT_KEY
default="INPUT"

3. When purging the directory, the input file will survive the purge. (Name of the surviving input file is based on the env var mentioned above)

Mainly to be aligned with CLI and Python implementation.
See: apify/apify-cli#960