Custom `baseUrl` and `stylePath` parameters

### Describe the problem

From @corrigancd in #140:

> Custom `baseUrl` and `stylePath` parameters - In our case, we require paths to the `itemId` that are not the ones hardcoded in the current implementation. So three parameters were added:
> * `baseUrl` - The path to where the `itemId` resides (defaults to the previous `portalUrl + ` if not specified and `useCustomUrlOrPath` (see below) is not passed as true);
> * `stylePath` - The path to the style json. This will be appended to after the `itemId` in the path when retrieving the vectorTileStyle file (defaults to the previous `` value if not specified and `useCustomUrlOrPath` is not passed as true);
> * `useCustomUrlOrPath` - Whether to use baseUrl and stylePath at all. If this is not passed, or passed as false, the previous functionality will happen, this includes the optional use of `portalUrl`.

### Describe the proposed solution

Proposed solution from From @corrigancd in #140: [here](https://github.com/Esri/esri-leaflet-vector/pull/140/files#diff-66fb4bf49f9bbda7e2b3c42d48d3a59f06d6426713a19d13787d567169e11b3dR43-R121) and [here](https://github.com/Esri/esri-leaflet-vector/pull/140/files#diff-d87a7a0181c4fb7f5a10b32e3494597f82c320b55306aa848df45d586513b30eR12-R21)

Start by reviewing the proposed changes linked from pull request #140 and trace the current URL construction for the itemId and vectorTileStyle file. The work is done when baseUrl, stylePath, and useCustomUrlOrPath support the described custom paths while preserving the existing defaults and optional portalUrl behavior.

Written by the indexing model from the issue text.