feat: error when using insecure identity on mainnet

[lwshang]

SDK-1904

Description

This used to be a warning. A hard error can abort the command so that no insecure state will be on the mainnet.

Users can surpress this error by setting export DFX_WARNING=-mainnet_plaintext_identity.

The warning won't display when executing commands like dfx deploy --playground.

$ dfx deploy --ic
Error: Failed to create AgentEnvironment.
Caused by: Failed to create AgentEnvironment for network 'ic'.
Caused by: The default identity is not stored securely. Do not use it to control a lot of cycles/ICP.
- For enhanced security, create a new identity using the command:
    dfx identity new
  Then, specify the new identity in mainnet-facing commands with the --identity flag.
- If you understand the risks and still wish to use the insecure plaintext identity, you can suppress this warning by running:
    export DFX_WARNING="-mainnet_plaintext_identity"
  After setting this environment variable, re-run the command.

$ dfx deploy --playground
Deploying all canisters.
Reserving canisters in playground...
Reserved canister 'hello_backend' with id zrakb-eaaaa-aaaab-qacaq-cai with the playground.
Reserved canister 'hello_frontend' with id 6ldcj-gyaaa-aaaab-qacsa-cai with the playground.
Building canisters...
Building frontend...
Installing canisters...
...

How Has This Been Tested?

e2e tests in network.bash and identity.bash

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.