Expose `PrimaryHostCoordinator` lease timeout and renewal interval as optional host configuration

## Summary

Azure Functions enables `PrimaryHostCoordinator` and maintains a Blob lease at:

`azure-webjobs-hosts/locks//host`

The current behavior is effectively:

- `LeaseTimeout = 15 seconds`
- `RenewalInterval = 12 seconds`

For a continuously active Function App, this produces one `RenewBlobLease` approximately every 12 seconds.

I propose exposing `PrimaryHostCoordinatorOptions.LeaseTimeout` and `PrimaryHostCoordinatorOptions.RenewalInterval` as **optional customer configuration**, while preserving the existing defaults unless explicitly overridden.

## Evidence

On a Flex Consumption Function App using managed identity/OAuth for `AzureWebJobsStorage`, Storage diagnostics show:

```text
OperationName: RenewBlobLease
URI: https://.blob.core.windows.net/azure-webjobs-hosts/locks//host?comp=lease
AuthenticationType: OAuth
UserAgent: azsdk-net-Storage.Blobs/12.22.1
Status: 200 Success
```

Measured cadence:

```text
~1 renewal / 12 seconds
~5 / minute
~300 / hour
~7,200 / day
~223,200 / 31-day month
```

This is from a single continuously active host, not scale-out multiplication.

In my account, these renewals are the dominant contributor by count to the Blob Storage `All Other Operations` meter and cost roughly €0.11–€0.12/month for one continuously active Function App. The amount is small per app, but it is fixed background overhead and scales with the number of continuously active apps.

## Existing runtime support

`PrimaryHostCoordinatorOptions` already exposes:

```csharp
public TimeSpan LeaseTimeout { get; set; }
public TimeSpan? RenewalInterval { get; set; }
```

with `LeaseTimeout` constrained to 15–60 seconds.

`PrimaryHostCoordinator` uses:

```csharp
RenewalInterval ?? LeaseTimeout.Add(TimeSpan.FromSeconds(-3))
```

So the runtime already supports configurations such as:

```text
LeaseTimeout = 60 seconds
RenewalInterval = 30 seconds
```

The missing piece is a supported Azure Functions configuration binding.

The existing `host.json` `singleton` settings do not apply here; they configure `SingletonOptions`, not the Primary Host Coordinator `/host` lease.

## Proposal

Expose these options through `host.json` and the standard app-setting override mechanism, for example:

```json
{
"primaryHostCoordinator": {
"leaseTimeout": "00:01:00",
"renewalInterval": "00:00:30"
}
}
```

with equivalent overrides such as:

```text
AzureFunctionsJobHost__primaryHostCoordinator__leaseTimeout=00:01:00
AzureFunctionsJobHost__primaryHostCoordinator__renewalInterval=00:00:30
```

The exact section name is not important.

The existing 15-second lease / 12-second renewal behavior should remain the default. Customers who opt into longer values would explicitly accept the corresponding failover tradeoff.

## Benefit

For one continuously active host:

| Configuration | Monthly renewals | Reduction |
|---|---:|---:|
| Current: 15s lease / 12s renewal | ~223,200 | — |
| 60s lease / 30s renewal | ~89,280 | **60%** |
| 60s lease / 57s renewal | ~46,990 | **79%** |

`60s / 30s` keeps more retry margin; `60s / 57s` minimizes transactions while preserving the current `LeaseTimeout - 3s` strategy.

Exposing both settings lets customers choose that tradeoff without changing existing defaults.

## Related issues

- Azure/azure-functions-host#2592 — Improve Host Lock Renewals
- Azure/azure-functions-host#11765 — Cost Regression due to added Health Check when using GPv2 Storage

#2592 already discusses this renewal strategy from a reliability perspective. This request is specifically to expose the existing coordinator options as an **opt-in customer setting** for workloads that want to choose their own storage-cost/failover tradeoff.

## Environment

```text
Plan: Azure Functions Flex Consumption
Runtime: Functions v4
Worker: Node.js
Storage: GPv2
Authentication: Managed Identity / OAuth
Observed lease: azure-webjobs-hosts/locks//host
Observed renewal cadence: ~12 seconds
```

Start by locating PrimaryHostCoordinatorOptions and PrimaryHostCoordinator, then trace how host.json and AzureFunctionsJobHost app-setting overrides are bound. Confirm the existing 15-second lease and 12-second renewal defaults, the allowed TimeSpan values, and how an opt-in configuration would preserve them when unset; done means both settings can be configured without changing current defaults.

Written by the indexing model from the issue text.