--bootstrap-XXXX options add instead of overriding

[schonex]

Describe the bug When using the --bootstrap-file or --bootstrap-node methods to start nimbus the provided node(s) are added to the list already hard coded into nimbus. There's no way with mainnet configuration to isolate nimbus and have it connect to nodes locally exclusively.

To Reproduce Steps to reproduce the behavior:

1. use above mentioned flags
2. get some popcorn
3. profit

Additional context As an aside, i don't think using file extensions is the right way to determine the format of the data provided. --bootstrap-file utilizes .txt or .enr before throwing an error about format. Not exactly user friendly way to inform the user what the 'real' problem is.

[kdeme]

There's no way with mainnet configuration to isolate nimbus and have it connect to nodes locally exclusively.

It should still be possible by manually adjusting/deleting the default bootstrap file. But I agree that this is not the most user friendly way and no documentation explains this.

Not exactly user friendly way to inform the user what the 'real' problem is.

I agree, the check on .txt or .enr is rather pointless and seems to confuse only more.

[tersec]

This is better done by copying the mainnet (or any other) configuration wholesale, modifying the bootstrap nodes, and using it as a runtime configuration. In general, certain types of customization are going to be available primarily via this mechanism.