Allow users to choose their own sanctions lists

## Summary

I propose that a new command line argument be added that allows users to set the sanctions list of their choosing. The default OFAC sanctions list would be removed.

## Proposed usage:

```sh
go run cmd/server/main.go -redis REDIS_URL -signingKey ETH_PRIVATE_KEY -proxy PROXY_URL -blocklist BLOCKLIST_PATH_OR_URL
```

## Example 1:

```sh
go run cmd/server/main.go -redis REDIS_URL -signingKey ETH_PRIVATE_KEY -proxy PROXY_URL -blocklist ~/sanctions.jsonc
```

```jsonc
/* filename: sanctions.jsonc */
[
"0xdead",
"0x1234",
"0x0" // You get the point
]
```

JSON-C was used as an example, but YAML, JSON, or just a newline-separated file could also be acceptable file formats. Do whatever is easiest to develop, as long as this feature is implemented.

## Example 2

```sh
go run cmd/server/main.go -redis REDIS_URL -signingKey ETH_PRIVATE_KEY -proxy PROXY_URL -blocklist https://someusgovmtwebsite.gov/blocklist.txt
```

URIs should be allowed as well as files.