在Kotlin中使用客户端证书进行HTTP请求，可以通过使用OkHttp库来实现。以下是一个示例代码：

import okhttp3.*
import java.io.File
import java.io.IOException
import java.security.KeyStore
import java.security.cert.CertificateFactory
import java.security.cert.X509Certificate
import javax.net.ssl.*

fun main() {
    val client = getClientWithCert()
    makeRequestWithCert(client)
}

fun getClientWithCert(): OkHttpClient {
    val certificateFile = File("path_to_certificate_file.crt")
    val keyStore = loadCertificateIntoKeyStore(certificateFile)

    val trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm())
    trustManagerFactory.init(keyStore)

    val trustManagers = trustManagerFactory.trustManagers

    val sslContext = SSLContext.getInstance("TLS")
    sslContext.init(null, trustManagers, null)

    return OkHttpClient.Builder()
        .sslSocketFactory(sslContext.socketFactory, trustManagers[0] as X509TrustManager)
        .build()
}

fun loadCertificateIntoKeyStore(certificateFile: File): KeyStore {
    val keyStore = KeyStore.getInstance(KeyStore.getDefaultType())
    keyStore.load(null)

    val certificateFactory = CertificateFactory.getInstance("X.509")
    val certificate = certificateFactory.generateCertificate(certificateFile.inputStream)

    keyStore.setCertificateEntry("my_cert", certificate)

    return keyStore
}

fun makeRequestWithCert(client: OkHttpClient) {
    val request = Request.Builder()
        .url("https://example.com/api/endpoint")
        .build()

    client.newCall(request).enqueue(object : Callback {
        override fun onFailure(call: Call, e: IOException) {
            e.printStackTrace()
        }

        override fun onResponse(call: Call, response: Response) {
            val responseBody = response.body?.string()
            println(responseBody)
        }
    })
}

在上述代码中，getClientWithCert()函数用于创建一个带有客户端证书的OkHttpClient实例。该函数首先加载证书文件到KeyStore中，然后使用TrustManagerFactory初始化TrustManager。接下来，使用SSLContext将TrustManager与SSL Socket Factory关联起来，创建一个带有SSL Socket Factory的OkHttpClient。

loadCertificateIntoKeyStore()函数用于将证书文件加载到KeyStore中。在该函数中，首先创建一个空的KeyStore实例，然后使用CertificateFactory生成证书，最后将证书添加到KeyStore中。

makeRequestWithCert()函数用于发起HTTP请求。在该函数中，首先创建一个Request对象，指定请求的URL。然后使用OkHttpClient的newCall()方法发送请求，并使用Callback处理响应的成功和失败情况。

请注意，上述代码中的path_to_certificate_file.crt需要替换为实际的证书文件路径。此外，还需要导入OkHttp库的依赖项。